Two-layer high temperature coating on a ceramic substrate, and process for obtaining same

ABSTRACT

The high temperature coating for the protection of ceramic materials, in particular porous ones, from erosion and chemical and/or mechanical failure according to the invention comprises a barrier (primer) layer and an emissivity glaze (topcoat) layer, with the following weight compositions: 
     barrier layer: 
     silicon tetraboride (with over 96 weight % of SiB 4 ): 0.1 to 10%; 
     quartz glass: 90 to 99.9%; 
     emissivity glaze layer: 
     silicon tetraboride (with over 96 weight % of SiB 4 ): 1.5 to 5.0%; 
     high silica glass: 95 to 98.5%.

This application is a divisional application Ser. No. 08/404,921, filedon Mar. 16, 1995 now U.S. Pat. No. 5,534,300.

FIELD OF THE INVENTION

The invention relates to materials, and more precisely to coatings, forthe protection of ceramic substrates, in particular porous ones, fromerosion and chemical and mechanical failure.

Refractory oxide based fibrous ceramic materials functioning at hightemperatures are widely used. The conditions under which these materialsare used place demands on the coatings used for their protection: highheat resistance, erosion resistance, thermochemical stability and phasestability.

BACKGROUND OF THE IN INVENTION

There is known at present a whole range of erosion resistant coatingswhich operate at temperatures of up to 1260° C. There is known atwo-layer coating comprising a barrier layer and a glaze layer (see U.S.Pat. No. 3,953,646). The barrier layer is formed by a deposit of fusedsilica comprising approximately 80 to 90% by weight of solid material.The coating is applied to the substrate by spraying. The barrier layeris fired at a temperature of approximately 930° to approximately 1370°C. The glaze layer, consisting of high silica glass, of borosilicateglass and of an emissivity agent, is applied to the barrier layer. Theemissivity layer is selected from the group formed by silicon carbide,chromium, cobalt and nickel oxides, nickel-chromium spinels, siliconnitride and calcined, mixed oxides of iron, chromium and/or nickel. Highsilica glass (Corning Glass No. 7913) contains not less than 94 weight %of SiO₂. The weight composition of the borosilcate glass (Corning GlassNo. 7740) is as follows: 70 to 87% of SiO₂, 10 to 20% of B₂ O₃, 2 to 5%of Na₂ O and 1 to 5% of Al₂ O₃.

The high silica glass component and the borosilicate glass component areused in a weight ratio of approximately 3:1 to approximately 19:1, andthe glass components (high silica glass and borosilicate glass) and theemissivity agent are used in a weight ratio ranging from 50:1 toapproximately 4:1. An aqueous slurry containing from approximately 10 toapproximately 90 weight % of glaze coating is fired at a temperatureranging from 930° to approximately 1370° C.

There is known in the art (see U.S. Pat. No. 3,955,034) athree-component coating for silica insulation comprising a silicabarrier layer, an emissivity layer comprising a high silica glasscomponent and an emissivity agent selected from the group formed bysilicon carbide, nickel oxide, chromium oxide, cobalt oxide, anickel-chromium spinel, silicon nitride and calcined, mixed oxides ofiron, chromium and cobalt, with a weight ratio of the high silica glassto the emissivity agent ranging from approximately 50:1 to approximately4:1, and an overglaze layer of high silica glass and borosilicate glassin a weight ratio of high silica glass to borosilicate glass rangingfrom approximately 3:1 to approximately 19:1. The coating is fired at atemperature ranging from 930° to approximately 1370° C.

These coatings provide neither sufficient thermal shock resistance norsufficient heat emissive stability, and they are subject to shrinkage.

To overcome the problems mentioned above, there has been proposed aone-layer coating (see U.S. Pat. No. 4,093,771) which is prepared byreacting a compound, selected from the group of substances formed bysilicon tetraboride, silicon hexaboride, other boron silicides, boronand mixtures of these substances, with a reactive glass frit composed ofhigh silica porous borosilicate glass and boron oxide. A thin layer ofborosilicate glass is formed on finely divided particles of high silicaglass, which improves the sintering of the coating without a substantialincrease in the thermal expansion coefficient.

The reactive glass frit is advantageously prepared by blendingapproximately 2 to 10 parts by weight of boron oxide with 100 parts byweight of high silica porous borosilicate glass, such as Vycon® 7930glass. Vycon® 7930 high silica borosilicate glass has a porosity ofapproximately 28%. The boron oxide is dissolved in 200 to 400 parts byweight of deionized water. The mixture is stirred at approximately 95°C., and then dried for a period of up to 24 hours, at a temperature of75° to 95° C. The resulting glass frit is dispersed, screened and firedat 1150° C. for 1 hour. The resulting sintered composite is ground to apowder and screened.

A typical composition would be 97.5 weight % of reactive glass fritcontaining 5.5 weight % of boron oxide, combined with 2.5 weight % ofsilicon tetraboride composed of 63±3 weight % of silicon, 36±3 weight %of boron and less than 0.2 weight % of magnesium. The coating slurry isprepared by blending finely divided particles of reactive glass frit andsilicon tetraboride, with a carrier such as ethanol and a pre-bindersuch as methylcellulose, in a proportion by weight of solid componentsof 35 to 50%. The mixture of coating components is milled in an aluminaball mill with alumina balls for 3 to 12 hours. The coating is appliedby spraying. The coated samples are dried for 2 to 5 hours attemperatures in the range of 20° to approximately 70° C. After drying,the coated samples are glazed in an oven for 1.5 hours at 1215° C. Thecoating has an emissivity of approximately 0.90 to 0.93 from ambienttemperature to over 1260° C. The thermal expansion coefficient is1.1•10⁻⁶ K⁻¹.

There is also known an advanced low density coating for the protectionof aluminosilicate porous materials that has an operating temperature ofup to 1300° C. The composition of the coating comprises 77.5 weight % ofreactive glass frit, 2.5 weight % of silicon tetraboride and 20 weight %of molybdenum disilicide. The coating is formed on the substrate at1230° C. for 1.5 hours (see: Advanced Porous Coating for low densityCeramic Insulation Materials, J. Amer. Ceram. Soc., vol. 72, No. 6,pages 1003-1010 , 1989).

The use of high silica borosilicate glass having an active surface inthe coating composition can lead to a reduction in thermochemicalstability and phase stability.

Phase transformations in the coating are linked with the formation ofα-cristobalite which causes cracking in the coating.

SUMMARY OF THE INVENTION

The present invention provides a high temperature coating on a ceramicsubstrate which does not have the drawbacks of the prior art coatings.The coating according to the invention has, in fact high thermochemicalstability, high resistance to thermal shocks, very stable phases and lowshrinkage.

More precisely, the invention provides a high temperature coating on aceramic substrate, in particular a porous one, characterized in thatthis coating comprises a barrier (primer) layer containing quartz glassand silicon tetraboride with over 96 weight % SiB₄, and an emissivityglaze (topcoat) layer including high silica glass and silicontetraboride with over 96 weight % of SiB₄, which layers have thefollowing weight compositions:

barrier layer:

silicon tetraboride (with over 96 weight % of SiB₄): 0.1 to 10%;

quartz glass: 90 to 99.9%;

emissivity glaze layer:

silicon tetraboride (with over 96 weight % of SiB₄): 1.5 to 5.0%;

high silica glass: 95 to 98.5%.

The ceramic substrate is made of a ceramic material generally comprisingone or more compounds selected from the group formed by Al₂ O₃, ZrO₂,SiO₂, SiC and Si₃ N₄. Its density is generally higher than 100 kg/m³(0.1 g/cm³).

In the coating, the content of silicon tetraboride particles the size ofwhich is less than 5 μm is preferably of 70 to 80 weight % of the saidsilicon tetraboride, the SiO₂ content of the quartz glass is preferably99.96 weight % and the high silica glass advantageously comprises, byweight: 94 to 96% of SiO₂ ; 3.5 to 6% of B₂ O₃ ; 0.1 to 0.5% of Al₂ O₃ ;and 0.1 to 0.5% of Na₂ O.

The thermochemical properties of the high silica glass due to itsspecific composition, in particular in silicon, aluminum and sodiumoxides, its high purity and its specific dispersity ensure the desiredchemical interaction between the amorphous (glass) and ceramicpolycrystalline (silicon tetraboride) components of the coating. As aresult, upon the firing of the ceramic substrate with its coating,curing takes place in its reactive emissivity glaze layer. This curingmakes it possible to reduce substantially the shrinkage of the ceramicsubstrate, to increase resistance to thermal shocks, and to increase thethermostability of the coating in order to stabilize the thermochemicalproperties.

The presence of over 98.5 weight % of high silica glass and of less than1.5 weight % of silicon tetraboride in the emissivity glaze layerincreases the softening temperature of the coating, which decreases itsphase stability and emissivity properties. If this layer comprises lessthan 95 weight % of high silica glass, and more than 5 weight % ofsilicon tetraboride, its heat resistance and thermochemical stabilityare insufficient.

A content of over 80 weight % of silicon tertraboride particles having asize of less than 5 μm leads to a substantial increase in its penetrancethrough the porous ceramic substrate, which leads to shrinkage of thelatter. A content of particles of this type of less than 70 weight %leads to non-uniform local distribution of silicon tetraboride throughthe glass matrix, which increases the stress on the coating and reducesits resistance to thermal shocks.

Humidity and dispersion of the silicon tetraboride are controlled. Ifthe quartz glass content of the barrier layer is greater than 99.9weight %, the density of the interface between the barrier layer and thesubstrate is too low, which reduces the adhesion between the coating andthe substrate and increases the penetrability of the emissivity glazelayer, which leads to shrinkage. If the quartz glass content is lessthan 90 weight %, the density of the barrier layer increasessubstantially, which leads to its non-uniform impregnation by thecomposition of the slurry intended to form the emissivity glaze layer,as well as to the appearance of cracking in the latter.

The use of high silica borosilicate glass, having an active surface, inthe coating composition could lead to a reduction in its thermochemicalstability and its phase stability.

Phase transformations in the coating are linked with the formation ofα-cristobalite, which causes cracking of the coating.

Penetration of the porous substrate by the coating confers highadhesion. A densified layer having a thickness of 70 to 140 μm and adensity of up to 500 kg/m³ (0.5 g/cm³) is generally formed on thesubstrate. The coating is advantageously applied using compressed air ata pressure of 0.8•10⁵ to 1.1•10⁵ Pa (0.8 to 1.1 atm). The dispersionphase (coating powder):dispersion medium (preferably distilled water)weight ratio is from 1:1 to 1:5. A high dispersion medium content, inparticular a high water content, leads to a non-uniform chemicalcomposition of the coating. A low water content leads to reduction inadhesion between the coating and the substrate. The coating is appliedto the surface of the ceramic substrate that has received a preliminarytreatment, for example by dedusting the felt forming the substrate, forbetter adhesion.

Firing the barrier (primer) layer at temperatures ranging from 1100° to1150° C. for 10 to 20 minutes and firing the emissivity glaze layer(topcoat) at temperatures ranging from 1250° to 1300° C. for 10 to 20minutes enable shrinkage of the material to be reduced.

The shrinkage of the coated material takes place at firing temperaturesof over 1300° C. and for firing times of over 20 minutes, whereas theemissivity properties decrease with temperatures and firing times belowthe values indicated hereabove.

According to another of its aspects, the invention thus relates to aprocess for providing a ceramic substrate, in particular a porous one,with the above-described coating, characterized in that it essentiallycomprises the steps consisting of:

preparing a first slurry of a powder consisting of 0.1 to 10 weight % ofsilicon tetraboride (with over 96 weight % of SiB₄) and 90 to 99.9weight % of quartz glass, in a compatible dispersion medium, preferablydistilled water, with a powder to liquid weight ratio of 1:1 to 1:5;

applying this first slurry, by spraying under pressure, onto the ceramicsubstrate to be coated, which has undergone a preparatory treatment;

drying the layer thus obtained and firing it at a temperature of between1100° and 1150° C. for 10 to 20 minutes, to obtain a barrier layer;

preparing a second slurry of a powder consisting of 1.5 to 5.0 weight %of silicon tetraboride (with over 96 weight % of SiB₄) and from 95 to98.5 weight % of high silica glass in a compatible dispersion medium,preferably distilled water, with a powder to liquid weight ratio of 1:1to 1:5;

applying this second slurry, by spraying under pressure, onto thebarrier layer formed previously;

drying the layer thus obtained and firing it at a temperature of 1250°to 1300° C. for 10 to 20 minutes, to obtain an emissivity glaze layer.

The following examples are intended to illustrate and more clearlyexplain the invention.

EXAMPLE 1

A coating was prepared using slurry coating-firing techniques.

Firstly, the barrier layer (primer layer) was prepared. Quartz glass wasmilled in an alumina ball mill to produce a powder having a specificsurface of 0.6 to 1 m² /g, and screened. 95 parts by weight of quartzglass and 5 parts by weight of silicon tetraboride comprising 70 weight% of particles having a size of less than 5 μm were blended in apolyethylene vessel for 25 hours. A weighed quantity of powder wasdiluted with distilled water in a weight ratio of 1:1 and applied byspraying at an air pressure of 10⁵ Pa (1 atm) onto the surface,pre-treated by dedusting, of a fiber based ceramic material. A densifiedlayer with a thickness of 100 μm was applied. The sample was dried inair for 30 minutes and in an oven at 80° C. for 30 minutes. The barrierlayer (primer layer) was then fired at 1120° C. for 15 minutes.

The cooling emissivity glaze layer (topcoat) was then applied to thebarrier layer.

The emissivity glaze layer was prepared from 95 parts by weight of highsilica glass and 5 parts by weight of silicon tetraboride comprising 70weight % of particles having a size of less than 5 μm. The glass wasmilled in an alumina ball mill to produce a powder having a specificsurface of 0.6 to 1 m² /g , and screened. The glass and silicontetraboride powders were blended in a polyethylene vessel for 48 hours.

A weighed quantity of powder was diluted with distilled water, in aweight ratio of 1:3, and applied to the barrier layer (primer layer) byspraying at an air pressure of 10⁵ Pa (1.0 atm). The sample was dried inair at 20° C. for 30 minutes and in an oven at 80° C. for 30 minutes.The emissivity glaze layer was fired at 1250° C. for 15 minutes.

EXAMPLE 2

The barrier layer was prepared according to the process of Example 1. Itcomprised 98 weight % of quartz glass and 2 weight % of silicontetraboride containing 75 weight % of particles having a size of lessthan 5 μm. The powder:water ratio was 1:2 and the air pressure was0.8•10⁵ (0.8 atm). The thickness of the densified layer obtained was 70μm. The barrier layer was fired at 1150° C. for 10 minutes.

The emissivity glaze layer was prepared according to the process ofExample 1.

EXAMPLE 3

The emissivity glaze layer comprised 98 weight % of high silica glassand 2 weight % of silicon tetraboride containing 80 weight % ofparticles having a size of less than 5 μm.

The emissivity glaze layer (topcoat) was applied to a barrier layer(primer layer) prepared according to the process of Example 2 and firedat 1280° C., for 10 minutes.

The coating was subjected to thermochemical and phase stability testsand to erosion resistance tests in a dissociated air flow. After 30cycles at 1250° C., the α-cristobalite content was not greater than 0.5weight %.

Thermochemical stability was evaluated by electron microscopy throughthe thickness of the porous layer (defective layer formed during thethermochemical stability test) the value of which was 30 μm. integralemissivity was 0.86 and the thermal expansion coefficient was 1.1•10⁻⁶K⁻¹. There was no shrinkage of the coating.

We claim:
 1. A high temperature coating on a ceramic substrate, whichcoating comprises a barrier layer as a primer layer and an emissivityglaze layer as a topcoat layer, wherein said barrier layer comprises 90to 99.9% by weight of a quartz glass and 0.1 to 10% by weight of asilicon tetraboride having a purity of greater than 96% by weight, andsaid emissivity glaze layer comprises 95 to 98.5% by weight of a highsilica glass and 1.5 to 5.0% by weight of a silicon tetraboride having apurity of greater than 96% by weight.
 2. The coating according to claim1, which is applied on a porous ceramic substrate.
 3. The coatingaccording to claim 1, wherein said barrier layer is densified and has athickness of 70 to 140 μm and a density of up to 0.5 g/cm³.